1. Field of the Invention
This invention relates to the field of semiconductor manufacturing, and more specifically, to a semiconductor transistor and its manufacture.
2. Discussion of Related Art
Various techniques are used to improve performance of a metal-oxide-semiconductor (MOS) transistor. Transistors have continuously been scaled down, thus increasing their density, and accordingly, their switching speeds.
Another way to increase the speed of the transistor, is to create a transistor channel with a high mobility of the carriers by, for example, growing a strained silicon (Si) layer on a relaxed silicon germanium (“Si1-x Gex”) layer thereby increasing mobility of electrons. The tensile strain in the Si layer, however, does not increase the hole mobility. Therefore, for a p-MOS transistor structure, the channel formed from the Si layer under tensile stress cannot provide increased hole mobility. In addition, the tensile strained Si layer is grown on a planar substrate, which limits the density of the transistors and increases intrinsic capacitance.